With increasing interest in packet data networks, and particularly wireless packet data networks, and with the increased use of packet data or electronic mail, transfer of data files, web browsing, digitized voice and video, more and more attention is being turned to the efficiency of such systems.
One aspect of such efficiency is the extent to which retransmission of data packets is required. When a data packet is corrupted in transit and needs to be retransmitted, it is retransmitted with a granularity which is determined by the structure of error detection information in the original packet. Some data protocols have a low degree of cyclical redundancy code (CRC) coding and must be transmitted in their entirety, where others have a high degree of CRC coding and allow much smaller degrees of retransmission. For example, a data packet such as a DataTAC.TM. protocol data unit or packet data unit (PDU) contains a single data CRC amounting to about one percent of the total packet size. When retransmission is required, the entire PDU must be retransmitted. Such a system is very efficient over a reliable channel, because of the low degree of redundant data. Efficiency falls away when extensive retransmission is necessary. By contrast, a Mobitex.TM. packet data system has two bytes of CRC every 18 data bytes amounting to ten percent of a total packet. In such a system, a portion of a complete packet can be retransmitted in the event of an error. Such a system has a high overhead but low retransmission cost and is less efficient when there is a good connection and more efficient when the channel is poor.
There is a need for an improved packet data communication system which is more efficient overall in a wide range of conditions.